When you plug in the USB and connect a DAC the M11 should recognise that a DAC is connected and the output should default to the USB. There is no output option in the settings menu, plug and play more or less.
The volume for the USB output will need to be set manually though, you should set the digital volume to maximum, I assume the M11S is the same as the Plus and maximum is 120, set it to that and adjust the final headphone volume at the analog stage obviously being the amplifier. If you disconnect and reconnect that USB audio device the M11 should recognise it again and set the volume to the same 120 as last time. Occasionally that doesn't work and you need to set the volume manually but that is not a big deal.
Over to you regarding the battery, I don't really understand what the charging management does and if trickling a battery constantly is better or worse for a battery than normal cycling.
Personally I would not take the chance with the battery life because the intended use case that you are describing always seems to be the one that is associated with bulged batteries after say 12 months. On all battery powered devices I typically run them from full to something close to flat and charge them again and I have never had a bulged battery and the battery lifetime on phones, battery powered DACs etc seems to be as good as can reasonably be expected so I have no reason to think that full cycling a battery is hard on it at all.
Using the M11 as digital transport with the screen brightness low or little use of the screen the battery will last literally 3 - 4 full days, I would just use it and let it drain and charge it overnight when needed.
